DE 10 2005 020 737 and corresponding U.S. application Ser. No. 11/127,856, DORNHOF, published as US 2005-025,3546-A, describe a method of sensorless operation of an electronically commutated motor having at least two phase winding strands, in which motor the voltage, induced in the non-energized stator phase winding strand of the stator, i.e. the strand not experiencing current flow at the moment, is differentiated, in order to generate a control signal for controlling commutation of the motor. Since, in stator winding arrangements having two strands, and in general in multi-strand stator windings, the stator winding strands never all experience current flow simultaneously, the sensing, of the zero transition of the induced voltage necessary for generation of the control signal, can always occur in a stator phase winding not experiencing current flow at the moment. This sensing requires, however, a relatively large outlay in terms of circuit engineering.
In a single-phase-winding motor, furthermore, current flow in the single stator phase winding strand must be discontinued during the expected zero transition of the induced voltage for a sufficiently long period of time to enable sensing of that zero transition. The result of such current-flow gaps is, however, to reduce the efficiency and maximum power of the motor.